More Major Hurricanes Coming This Century

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Strong hurricanes could hit Asia and the U.S. East Coast more
often this century, a new study finds.

The research adds to a growing body of evidence that hurricanes
are becoming more intense as global warming heats the oceans.
This means Category 1, 2 and 3 storms will have fiercer winds,
bumping them up to Category 3, 4 and higher. Overall, the study's
modeling approach predicts a 40 percent global increase in
tropical cyclones of Category 3 and higher during the 21st
century.

The findings were published in today's (July 8) issue of the
journal Proceedings of the National Academy of Sciences.

The new study also projects that these strong storms will become
more frequent in the North Pacific, the North Atlantic and the
South Indian Oceans.

"We see an increase, in particular, toward the middle of the
century," said Kerry Emanuel, study author and an MIT
climatologist. "The results surprised us, but we haven't gotten
so far as to understand why this is happening."

Scientists actively debate whether tropical cyclones (the broad
name for hurricanes, typhoons and tropical storms) will become
more
frequent, more intense or both as a result of climate change.
Global warming has heated the oceans along with the atmosphere,
and sea-surface temperature is one of the most important
influences on hurricane strength. But other factors — such as
climate cycles, like the El Niño Southern Oscillation, and
interactions with other storms, which can weaken or strengthen
storms — also alter tropical storm strength. Hurricanes are also
marvelous heat transporters, affecting climate by moving energy
around the planet. [ Hurricanes
from Above: See Nature's Biggest Storms ]

"For scientists, this is far from a solved problem," said
Emanuel, who has been a main participant in the debates on the
future of hurricanes. "The main message is, we have to continue
to regard there being a not-trivial risk of increasing problems
from tropical cyclones because of climate change."

Hurricanes feed off warm ocean water. In the ocean's hurricane
nurseries, heat rising from the ocean turns into water vapor. As
the vapor rises and cools, it condenses into rain. This releases
heat, which helps strengthen circulating tropical cyclones.
Warmer oceans mean more water vapor, and more intense storms.

Emanuel relies on a technique called downscaling to estimate how
future climate change will shift hurricane strength and
frequency. In the new study, he used the latest global
climate models, called CMIP5, which project future climate
change but are too coarse to resolve "small" features, such as
hurricanes. Emanuel ran the models at higher-resolution and
randomly generated disturbances similar to tropical cyclones, and
then used a theoretical model to predict how strong the storms
would become. Emanuel first presented this approach, with an
earlier version of the CMIP models (CMIP3), in a 2008 paper in
the Bulletin of the American Meteorological Society.

One limitation of relying on global climate models to predict
future hurricanes is that these models do not capture the past
decade's climate variability, such as relatively stable
temperatures between 1998 and 2008, said Peter Webster, a
climatologist at Georgia Tech who was not involved in the study.
(The hiatus is attributed to natural variability in Earth's
climate system, superimposed on the longer-term warming trend.)

"It should be remembered that the study is a model interpretation
of how things might be," Webster said. "So, elegant as [the
study] is, it is not going to tilt the scales on global warming
one way or the other. The scales will be tilted substantially
once we understand the role of tropical cyclones in climate and
how tropical cyclones, in turn, modify climate itself."

More storms in Asia, Atlantic Coast

In the new study, the North Pacific Ocean basin showed the
strongest change in tropical-cyclone frequency and intensity.
This means Asia and its global supply chains will be hit hardest
by the changes, with higher-intensity storms striking more often,
said Matthew Huber, a climatologist and director of the Purdue
University Climate Change Research Center who was not involved in
the study. [ How
Strong Can a Hurricane Get? ]

In addition, the model predicts that the North Atlantic and South
Indian Oceans will also see an increase in storm frequency and
intensity. This contradicts Emanuel's earlier research and other
studies based on the previous generation of climate models, which
predicted fewer but stronger hurricanes in the North Atlantic,
the basin where storms that threaten the East Coast and Southeast
form. However, a recent
study of coastal storm surge records in the southern United
States supports Emanuel's new data.

But Huber said Emanuel's results for the East Coast are not well
supported by previous research and should be viewed as less
certain.

But something the scientists do agree on is that coastal cities
need to improve their defenses, as sea level rise alone will
increase the vulnerability of such areas to storm surge.

"The result represents a significant upward revision of previous
estimates of tropical cyclone activity in a warmer world, so it
is unlikely that communities and states are prepared for, or even
preparing for, the magnitude of future risks appropriately,"
Huber said.